Drum mixers for manufacturing asphaltic compositions out of an aggregate material are known in the art. An aggregate material known as xe2x80x9crecycled asphaltic pavementxe2x80x9d (RAP) is an inexpensive and plentiful aggregate material which can be used to manufacture an asphaltic composition. The RAP material is formed from a mixture of an asphaltic material, aggregates and mineral binder or xe2x80x9cfinesxe2x80x9d.
Virgin aggregates can also be used in manufacturing asphaltic compositions. As the virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and fines to produce the asphalt composition. However, producing an asphaltic composition from virgin aggregate is more expensive than producing the asphaltic composition from RAP material because the virgin aggregate is more costly than the RAP material, and more asphaltic material must be added to the virgin aggregate.
When RAP material has been used in previous drum mixers, the RAP material was introduced into the drum mixer in a different location separate from the virgin aggregate to minimize what is known in the art as xe2x80x9cblue smokexe2x80x9d and also to not degrade the RAP material. And, as a practical matter, the ratio of RAP material which could be used relative to virgin aggregate was about 25% with maximums up to 50% in some cases. Thus, it has been necessary to use a substantial amount of expensive virgin aggregate in producing the asphaltic composition.
By increasing the ratio of RAP material to virgin aggregate, the costs of manufacturing the asphaltic composition can be significantly reduced. It is to such a drum mixer for manufacturing an asphaltic composition out of a high ratio of RAP material to virgin aggregate material that the present invention is directed.
The present invention is a drum mixer for heating, mixing and drying an aggregate material, such as a mixture of recycled asphaltic pavement and virgin aggregate. The drum mixer includes an inclined drum having a first end and a second end. The drum forms a pre-heating/blending section adjacent the first end, a heating/mixing section adjacent the second end, a heating/drying/mixing section therebetween, and a discharge outlet adjacent the second end.
A feed assembly for feeding aggregate material is provided. The feed assembly feeds the aggregate material into the pre-heating/blending section of the drum for movement of the aggregate material sequentially through the pre-heating/blending section, the heating/drying/mixing section, the heating/mixing section, and the discharge outlet. A burner assembly extends from the second end of the drum into the heating/drying/mixing section of the drum. The burner assembly creates a high temperature gas stream which flows through the heating/drying/mixing and preheating/blending sections of the drum.
Tubular compartments are positioned in the heating/drying/mixing section of the drum so as to form a plurality of aggregate transporting channels. The aggregate transporting channels within the tubular compartments are out of direct contact with the high temperature gas stream while the tubular compartments are exposed to the high temperature gas stream. Thus, the aggregate material is heated, and dried indirectly via conduction of heat through the tubular compartments as the aggregate material passes through the aggregate transporting channels.
In some aspects of the present invention, the drum mixer may also include one or more fluid injectors and fines injectors, both of which communicate with the drum for injecting an asphaltic fluid, fines or other additives, such as fibers or anti-strip agents into the drum so that the asphaltic fluid, fines or other additives are mixed with the aggregate material to form the asphaltic composition.